Sure it will work as long as you have a good ground. But it will charge all the time its hooked up. Gotta watch overcharging/heating.
Also you need a pretty heavy wire to carry the charge.Sorry I dont know one guage from another but light wire will get HOT FAST.

You shouldn't have to worry about over charging. The Jeeps voltage regulator will take care of that. But if your using anything like a 12 volt Ref. unplug it from the jeep when you stop for anything. If not both batteries will be drawn down. How fast? Duno, just unplug them, its safer that way.